1. Field of the Invention
This invention relates to an insulated valve for an internal combustion engine and more specifically to a fillet heat shield for a poppet valve in such an engine.
2. Description of the Prior Art
Shields or thermal barriers for protecting valves in piston engine combustion chambers are well-known. For many years such shields or barriers have been proposed for poppet type exhaust valves to reduce heat corrosion and weakening of valves and to reduce heat flow from combustion gases to the valves. For example, U.S. Pat. No. 1,868,138 issued to E. J. Fisk in 1932 discloses several thermal barrier embodiments which cover the face portion of an exhaust valve to insulate or shield the valve from combusting gases in the combustion chamber. U.S. Pat. No. 1,727,621 issued to A. Taub in 1929 discloses several thermal barrier embodiments which cover the fillet and part of the stem of an exhaust valve to insulate or shield the valve from combusted gases when the valve is open. Neither of these patents or analogous patents teach or suggest any advantages in shielding both the valve face and fillet. Further both of these patents are concerned with inhibiting heat flow into a valve.
Analytical studies and testing in connection with the instant invention on one particular diesel engine indicates that (1) the fillet of an exhaust valve without a shielded face operates at substantially the same average temperature as the exhaust gases flowing over the fillet, (2) a fillet heat shield on an intake valve without a shielded face can reduce about 65 percent of the heat flowing from the valve fillet and stem to the incoming air or air fuel mixture, and (3) a fillet heat shield or an intake valve with a shielded face can reduce about 90 percent of the heat flowing from the valve fillet and stem to an incoming air or air fuel mixture. The first of these three items indicates that for this particular engine there is little or no advantage in shielding only the fillet portion of an exhaust valve. Hence, a fillet heat shield on an exhaust valve not having a face shield is of little value. The second and third items indicate that a fillet heat shield on an intake valve or a fillet and face shield on an intake valve can substantially reduce heat flow from the valve fillet to the incoming air or air fuel mixture, whereby a cooler intake charge is ingested by the combustion chamber with a resultant increase in the amount of ingested charge, with a resultant reduction in NO.sub.x which decreases logarithmically with lower temperatures, and with a reduction in octane requirements which decrease with decreasing temperatures.
Further, since the surface of the fillet shield is substantially cooler than the fillet surface of an unshielded valve, coking of the fillet by a fuel-air charge is inhibited.
In addition to the above, face heat shields can significantly reduce fuel consumption and improve exhaust emission quality during engine warm-up, since the face shields warm up faster than the faces of unshielded valves.
Further, valves with both face and fillet heat shields reduce heat flow to the engine structure and cooling system via valve contact with the engine structure, whereby additional power may be extracted from the exhaust gases when the engine is turbo-charged and/or turbocompounded.